The present invention relates to transducers of the type employed for providing a continuous electrical indication in real time of the chemical/contaminate condition of a fluid such as hydraulic fluids and for additive depletion, acidity and other types of degradation of lubricants employed in internal combustion engines and power transmission devices. An example of such a device employing impedance spectroscopy techniques for fluid monitoring is that shown and described in co-pending application Ser. No. 220,556, filed Dec. 23, 1998 entitled xe2x80x9cFluid Condition Monitorxe2x80x9d, now U.S. Pat. No. 6,278,281, and assigned to the Assignee of the present application. The aforesaid device utilizes a probe having interdigitated elements immersed in the fluid and excited by an alternating current signal sequentially at a low frequency and at a higher frequency. The resulting voltage is measured during excitation at both frequencies and the impedance is computed and the difference in impedance between the high and low frequency utilized as an indication of the fluid condition which can be determined from a look-up table of differential impedance measurements taken of fluids having a known condition. The sensing device or transducer has heretofore employed a probe having a pair of electrodes disposed in closely spaced generally parallel arrangement and immersed in the fluid with electrical connections made thereto such that one electrode is excited by the alternating voltage sequentially at the selected frequencies; and, the sensed or output current from which the impedance determinations are made flows through and is detected in the remaining electrode.
Referring to FIG. 6, a known system for determining fluid conditions such as that described in co-pending application Ser. No. 220,556, filed Dec. 23, 1998 entitled xe2x80x9cFluid Condition Monitorxe2x80x9d in the names of R. A. Bauer et al. and assigned to the Assignee of the present invention is illustrated. The known system of FIG. 6 employs the technique of sequentially exciting a probe having a pair of electrodes immersed in the fluid with alternating excitation voltage at low and high frequencies such as 0.01 hertz and 10 hertz respectively and determining the resultant current flow in the other electrode and computing the difference in impedance arrived from the measured currents. The system as indicated generally at 1 in FIG. 6 and employs an excitation electrode 2 immersed in the fluid to be monitored with a current sensing or output electrode 3 disposed in closely spaced generally parallel interdigitated arrangement. The electrode 2 receives excitation voltage through a shielded lead 4 which receives an analog excitation signal from an electronic controller (not shown in FIG. 6) applied through a level shifter 5. The output or current sensing electrode 3 is connected via a shielded lead 6 of the input of a current to voltage converter 7 the output of which is applied through a level shifter 8 connected to the input of an analog to digital converter provided in the unshown electronic controller. The current to voltage converter 7 has a variable resistor RF which is varied by an auto range control signal received from the unshown controller along 9. The signal processing for the system is sufficiently shown and described in the aforesaid co-pending application and the details thereof are incorporated herein by reference and not repeated for the sake of brevity.
A separate set of level sensing electrodes 12, 13 is immersed in the fluid and connected by leads 14, 15 to the unknown controller. The prior art system of FIG. 6 includes a separate temperature sensing device such as a thermistor denoted by reference numeral 10 immersed in the fluid and which provides a signal through line 11 to temperature sensing signal processing circuitry provided in the unshown electronic controller.
It is also known to provide the prior art system of FIG. 6 with an alternative configuration for the arrangement of the probe electrodes in the form of a spiral or helically disposed spaced parallel wires in place of the interdigitated electrodes 2, 3 as shown in FIG. 6. Such a helical configuration for the probe is shown and described in co-pending application 09/432,971, filed Nov. 3, 1999 entitled xe2x80x9cMonitoring Fluid Condition With A Spiral Electrode Configurationxe2x80x9d and filed in the names of M. H. Polcyznsky et al. and assigned to the Assignee of the present invention.
Heretofore, where it was also desired to provide an electrical indication of the fluid level in the reservoir or vessel for which the condition sensor or monitor was being employed, it has been necessary to provide an additional electrode set in the fluid and to provide support therefore in spaced relationship to the fluid condition sensing electrodes and to provide separate electronic circuitry for electrically determining the fluid level. This arrangement has proven relatively costly and has complicated the construction of the fluid condition monitoring probe assembly and installation of same, particularly where the fluid to be monitored is contained in a reservoir or vessel which is required to be sealed.
In particular where it is desired to employ the fluid condition monitoring sensor on board a motor vehicle for providing a continuous electrical indication of the condition of the fluid in either the transmission or the engine oil sump, the complications of adding an additional electrode for fluid level sensing have been considered prohibitive in high volume mass production from a cost and complexity of installation standpoint. Accordingly, it has long been desired to provide a way or means of sensing the fluid level in the reservoir vessel for which a fluid condition monitor is employed and to provide an electrical indication of the fluid condition without the need for separate fluid level sensing electrodes.
The present invention solves the above described problem of providing a combined fluid condition sensing function with fluid level sensing in a common sensor probe without the need for a separate electrode and circuitry for the sensing function. The sensor of the present invention utilizes a pair of closely spaced generally parallel electrodes immersed in the fluid with one of the electrodes divided into an upper and lower section which are electrically connected together and commonly excited by an alternating current sequentially at a high and low frequency for determining the fluid condition. The remaining electrode is the sensing electrode and provides a signal current to the electronic controller for computation of the differential impedance at the high and low frequencies of excitation for determination of the fluid condition from stored information.
For the level sensing function the excitation electrode is divided into upper and lower sections which are excited separately; and, the ratio of the currents and signal phase shifts determined from excitation of each section provides an indication of whether both electrodes are immersed in fluid, e.g., that the fluid is at the desired level. If either electrode is above the fluid level a significant increase in phase shaft is detected. The ratio of the current may be used to indicate the proportion of the electrodes immersed to thereby determine intermediate levels of the fluid. The present invention thus combines fluid level sensing with fluid condition monitoring in the same probe by segmenting the excitation electrode and eliminates the need for providing a separate set of electrodes for the level sensing function.